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Exposed subsurface ice sheets in the Martian mid-latitudes

Thick deposits cover broad regions of the Martian mid-latitudes with a smooth mantle; erosion
in these regions creates scarps that expose the internal structure of the mantle.We
investigated eight of these locations and found that they expose deposits of water ice that
can be >100 meters thick, extending downward from depths as shallow as 1 to 2 meters below
the surface.The scarps are actively retreating because of sublimation of the exposed water
ice.The ice deposits likely originated as snowfall during Mars’ high-obliquity periods and have
now compacted into massive, fractured, and layered ice.We expect the vertical structure of
Martian ice-rich deposits to preserve a record of ice deposition and past climate.

Exposed subsurface ice sheets in the Martian mid-latitudes

1.
REPORT
◥
MARTIAN GEOLOGY
Exposed subsurface ice sheets in the
Martian mid-latitudes
Colin M. Dundas,1
* Ali M. Bramson,2
Lujendra Ojha,3
James J. Wray,4
Michael T. Mellon,5
Shane Byrne,2
Alfred S. McEwen,2
Nathaniel E. Putzig,6
Donna Viola,2
Sarah Sutton,2
Erin Clark,2
John W. Holt7
Thick deposits cover broad regions of the Martian mid-latitudes with a smooth mantle; erosion
in these regions creates scarps that expose the internal structure of the mantle. We
investigated eight of these locations and found that they expose deposits of water ice that
can be >100 meters thick, extending downward from depths as shallow as 1 to 2 meters below
the surface.The scarps are actively retreating because of sublimation of the exposed water
ice.The ice deposits likely originated as snowfall during Mars’ high-obliquity periods and have
now compacted into massive, fractured, and layered ice. We expect the vertical structure of
Martian ice-rich deposits to preserve a record of ice deposition and past climate.
O
ne-third of the Martian surface contains
shallow ground ice. This ice is a critical
target for science and exploration: it affects
modern geomorphology, is expected to pre-
serve a record of climate history, influences
the planet’s habitability, and may be a potential
resource for future exploration. The extent of
Martian ground ice and the depth to the ice table
have been predicted in theory (1–3) and have
been tested both in situ (4) and from orbital ob-
servations (5–11). However, the vertical structure
of subsurface ice remains poorly known, includ-
ing its layering, thickness, and purity, which
record its emplacement and subsequent modifi-
cation processes. Information about the structure,
depth, and purity of shallow ice is also required to
plan possible in situ resource utilization (ISRU) on
future missions (12).
Early theoretical predictions suggested that
Martian subsurface ice would be ice-cemented
ground (2). Orbital neutron-spectrometer data
have revealed ice contents greater than the likely
pore space volume in the upper few centimeters
of the ice table in many locations (5–7). Shallow
ice (<1 to 2 m) exposed by fresh impacts remains
distinct for months or years, also indicating a low
rock or dust content, although possibly modified
by the impact process (9, 10, 13). The Phoenix
lander on the northern plains uncovered both ice-
cemented regolith and deposits of pure (~99 vol-
ume %) ice (4, 14) at a few centimeters depth.
The ice at that site likely extends to 9 to 66 m
depth on the basis of shallow radar reflections
(15), but geomorphic interpretations suggest that
ice-cemented ground is dominant there (16).
However, ice-loss landforms indicate that several
regions on Mars have high ice volume fractions
extending through substantial subsurface depths
(17, 18), and radar echo power data have sug-
gested high ice contents beneath areas of the
northern plains (8). Subsurface radar reflections
indicate the presence of debris-covered glaciers
(19, 20) as well as buried regional ice sheets in
the Utopia and Arcadia Planitiae regions that are
up to 170 m thick and nearly pure ice (21, 22).
Because the radar did not resolve the top of the
ice, it is likely to be within ~20 m of the surface,
the limit of the radar instrument’s ability to iden-
tify shallow signals (15). The smaller-scale structure
of the ice sheets and rocky cover are unresolvable
with current radar data. It remains unknown
whether the ice within a few meters of the sur-
face has the same origin and age as the deeper
ice because the upper ice is most readily modi-
fied and best coupled to the recent climate.
We describe observations of the vertical struc-
ture of ground ice using the Mars Reconnaissance
Orbiter (MRO). The observations target eight loca-
tions that have steep, pole-facing scarps created by
erosion (Figs. 1 and 2 and figs. S1 to S3) in images
from the High-Resolution Imaging Science Exper-
iment (HiRISE); seven are located in the southern
hemisphere, and the eighth location is a cluster of
scarps in Milankovič Crater in the north (table S1).
Each of the scarps is relatively blue (compared with
surrounding terrain) in enhanced-color HiRISE
images, and three locations have water-ice signa-
tures in mid-summer spectral data (Fig. 3 and
figs. S4 and S5) taken by MRO’s Compact Recon-
naissance ImagingSpectrometerforMars (CRISM)
(23). CRISM spectra show an H2O ice absorption
feature at 1-mm wavelength (fig. S6), which can
be masked by as little as 1% soil (14). Tens of micro-
meters of dust are needed to make an opaque cover
(24), so the dust content is low and/or wind re-
moves dust.
Several lines of evidence indicate that the scarps
are exposures of subsurface ice rather than per-
sistent seasonal frost. First, they remain distinct
from the surrounding terrain in color in all HiRISE
images, including many observations acquired
long after seasonal frost has sublimed from steep
pole-facing slopes at higher latitudes. Second, Mars
Odyssey Thermal Emission Imaging System
(THEMIS) observations indicate late-afternoon
scarp temperatures above the likely atmospheric
frost point (23). Last, nearby pits lack relatively
blue material that would be expected for topo-
graphically controlled frost (fig. S7).
The units hosting the scarps drape the terrain,
with some meter-scale boulders on the surface
RESEARCH
Dundas et al., Science 359, 199–201 (2018) 12 January 2018 1 of 3
1
Astrogeology Science Center, U.S. Geological Survey, 2255 N.
Gemini Drive, Flagstaff, AZ 86001, USA. 2
Lunar and Planetary
Laboratory, University of Arizona, Tucson, AZ, USA.
3
Department of Earth and Planetary Sciences, The Johns
Hopkins University, Baltimore, MD, USA. 4
School of Earth and
Atmospheric Sciences, Georgia Institute of Technology, Atlanta,
GA, USA. 5
The Johns Hopkins University/Applied Physics
Laboratory, Laurel, MD, USA. 6
Planetary Science Institute, 1546
Cole Boulevard, Suite 120, Lakewood, CO 80401, USA. 7
Institute
for Geophysics, Jackson School of Geosciences, University of
Texas at Austin, Austin, TX 78758, USA.
*Corresponding author. Email: cdundas@usgs.gov
Fig. 1. Pits with scarps exposing ice. (A and B) Scarps 1 and 2. Both (A) and (B) show HiRISE
red-filter data merged with the center color strip (23) in early-summer observations. Parallel ridges
indicate retreat of scarps (fig. S1). North is up and light is from the left in all figures.
onJanuary13,2018http://science.sciencemag.org/Downloadedfrom

2.
and few impact craters. The surface textures show
some fracturing (fig. S8), probably due to thermal
contraction. Ice is expected to be stable near 10 cm
depth under level ground at these locations (3).
The scarps are sharply defined and nearly straight,
up to ~6 km long, and face slightly east of pole-
ward. Each scarp constitutes the equatorward
side of a few-kilometer-scale pit with a rugged
and sometimes boulder-rich floor, except in
Milankovič Crater, where they are part of the
ragged edge of a smooth plateau (fig. S2). In several
cases, ridge structures parallel the scarp within
the pit and merge with cusps along the margin.
The base of the exposed ice appears to be covered
by colluvium (Fig. 2). In several cases, surface
expressions of long fractures with surrounding
depressions occur on the terrain above the scarp
(fig. S8).
Topographic data (figs. S9 and S10) for Scarp 1
(numbering is provided in table S1) indicates
that the ice exposure has a slope of ~45° (up to
55° in some locations), transitioning to a shal-
lower slope for the regolith-covered lower scarp.
We estimate the scarp-hosting material to be at
least 130 m thick after correcting for the regional
slope, although the exposed ice has ≤100 m of relief
because the lower slope is buried. The former value
provides a better estimate of the ice-rich thickness
if all of the relief is due to ice loss.
These scarps reveal the vertical structure of the
ice. At several locations, ice reaches close to the
top of the scarp (Fig. 2), indicating that massive
ice begins at shallow depths (≤1 m). Banded
patterns or variations in color or slope at some
sites suggest subunits within the ice. At Scarps 6
and 7, the banded patterns show unconformities
where layers cut across each other (fig. S3). The
uppermost section of the scarp at each site ap-
pears steeper and less blue than the remainder.
Like the Martian polar deposits (25), the color
and brightness of the surface may be partially
controlled by a thin coating of dust (likely a sub-
limation lag) and thus not representative of the
bulk properties. Because lags and contaminants
will reduce the visibility of ice, it is likely that
clean ice near the surface is more extensive than
the limited locations where it is exposed (Fig. 2).
At Scarp 2, a lens-shaped section of the ex-
posure lacks relatively blue coloration (Fig. 4A)
and contains a concentration of boulders. Repeat
images taken 3 Mars years apart show that meter-
scale blocks fell from within or below this lens-
shaped section(Fig. 4, B and C), providing evidence
of active slope retreat. Because these blocks did
not disappear after they fell [unlike ice blocks
excavated by craters (10)] and originated as pro-
trusions from the icy layer, they must be rocks
that were embedded within the deposit, not chunks
of ice broken off the scarp. Other sites have not
shown similar blockfalls, but mottled shifts in
tone and texture are common, and dark mark-
ings a few meters across appeared and disappeared
at the base of some scarps (fig. S11).
The Shallow Radar (SHARAD) on MRO has
detected radar contacts (reflectors) that have been
interpreted as the base of ice layers in the northern
plains of Mars (15, 19–22). Similar features occur
near several of the scarps (fig. S12) but are dif-
ficult to distinguish from radar clutter caused by
local rugged topography. Candidate reflectors in
the three SHARAD tracks closest to Scarp 1 do
not appear in clutter simulations by using topo-
graphy derived from either Mars Orbiter Laser
Altimeter (MOLA) or High-Resolution Stereo
Camera (HRSC) data (23), suggesting that they
could be due to the bottom of the massive ice
layer. However, candidate reflectors near Scarp 5
have possible counterparts in the HRSC-based
clutter simulations, suggesting that they are not
subsurface features. Only these two sites have
topographic data available at suitable resolu-
tion, and given the rough terrain, we cannot
currently rule out unresolved clutter as the cause
of candidate reflectors. Nondetections may be
due to surface roughness or stratigraphic con-
tacts that are gradual or otherwise do not pro-
duce a sharp dielectric contrast on the scale of
the radar footprint.
We interpret these scarps as exposures of sub-
surface ice originating as dusty snow or frost
(26–29) and subsequently compacted and recrys-
tallized. This interpretation is consistent with the
high ice content and the mantling appearance of
the host unit. Alternative explanations, such as
growth of ice lenses (30) or enhanced vapor dif-
fusion (31), are expected to be slow and operate
at shallow depths. They should also produce layer-
ing parallel to the ground surface, unlike the layers
at Scarps 6 and 7 (fig. S3). The latest ice deposition
could have been geologically recent because there
are few craters on the surface. The fractures and
steep slopes indicate that the ice is cohesive and
strong. The presence of banding and color varia-
tions suggest layers, possibly deposited with
changes in the proportion of ice and dust under
varying climate conditions, similar to the Martian
polar-layered deposits (25). The lens-like body of
lower ice content at Scarp 2 (Fig. 4A) may be a
buried moraine transported by ice flow because
subtle arcuate surface topography suggests gla-
cial flow at this location (fig. S13). Alternatively, it
could be a former sublimation lag; that would
require some additional process to explain the
presence of boulders, but some are found on the
surface of the mantling deposit elsewhere (23),
so this cannot be ruled out.
It is likely that the scarps are currently re-
treating owing to sublimation. Slope retreat caused
by sublimation would explain rocks falling from
Scarp 2, although the proximate cause could be
seasonal frost processes or thermal cycles. Be-
cause the boulders that fell from the scarp are
meter-scale, with a few percent of the boulders
present having fallen over an interval of 3 Mars
years, the likely retreat rate is on the order of a few
millimeterseachsummer(unresolvablewithremote
imagery). This estimated retreat rate indicates that
the pits may have formed over a time scale on an
order of 106
years; however, the retreat rate is
probably not constant. Ridges and bands paral-
leling the scarps may indicate former positions
(Fig. 1).
Dundas et al., Science 359, 199–201 (2018) 12 January 2018 2 of 3
Fig. 2. Enhanced-color transverse sections of icy scarps in late spring/
early summer. (A to F) Arrows indicate locations where relatively blue
material is particularly close to the surface. Downhill is to the bottom in (A) to
(E) (Scarps 1 to 5, respectively) and to the top in (F) (Milankovič Crater).
Banding or layering is visible in several scarps, and (E) shows a distinct
change in color as well as multiple fractures cutting the ice.
RESEARCH | REPORT
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